Some restaurants will substitute tobiko with masago, which is the slightly smaller capelin roe, because it’s cheaper and looks similar.
Beyond being so pleasant to the eye, flying fish roe has a subtle but delicious flavor and texture. It’s smaller and grittier than salmon roe (called “ikura” when cured via the common Japanese method with soy sauce, dashi, and sake), so has an almost crunchy popping texture. Along with this crunch, tobiko also has a mild, slightly tangy flavor that tastes like a little dose of salt and sea.
You can find canned flying fish eggs near Uwajimaya’s seafood department.